The present invention relates to apparatus for measuring vehicle wheel speeds in accordance with digital values measured within the time duration of the cycles of an alternating voltage generated by means of a toothed rotor which may be of the type typically known as a magnetic pick-up.
Such wheel speed measuring apparatus is used in conjunction with anti-skid brake control systems for preventing the occurrence of an inadmissible slip of the vehicle wheel, which is the incipient condition of wheel skids.
Because of difficulties experienced during manufacture of the toothed rotor member, close dimensional tolerances between a tooth and tooth space cannot be achieved economically. In actual service, these dimensional variances result in signal distortion to the extent that different measured values of the half-cycle durations comprising the generated sinusoidal voltage waveform developed. In that these digital measures of the successive half-cycle durations of the generated sinusoidal voltage are used to detect wheel speed, any difference in the measured values will indicate a change of wheel speed, which is in turn used to detect wheel slips. It is imperative, therefore, that in order to prevent false wheel slip signals from occurring, signal distortions resulting from the dimensional tolerances of the toothed rotor member must be eliminated.
This can be accomplished by sensing only successive full cycles, or by sensing only the positive or the negative half cycles of the generated sinusoidal voltage. In the former instance, the error due to the measured difference between successive half cycles is averaged out over the full cycle, while in the latter case the half cycle measures are taken only from the teeth or from the tooth spaces in order to avoid measuring any dimensional differences between a tooth and tooth space. By avoiding signal distortions in this manner, however, it becomes apparent that only a single unit of information is utilized by measuring either the single full cycle or a single half cycle of the sinusoidal voltage, whereas each half cycle of the sinusoidal voltage makes possible the use of two information units per cycle. This lack of full utilization of available information units results in inefficient wheel skid control, particularly at low vehicle speeds.
In copending U.S. patent application, Ser. No. 733,424 (Case 6849), filed Oct. 18, 1976, there is disclosed an electronic circuit arrangement in which the measured time values of successive half cycles of a sinusoidal voltage generated by the alternate passing of a tooth and tooth space of a rotor in proximity of a magnetic pick-up device are compared and any error difference is corrected in order to obtain accurate wheel speed signals, without loss of efficiency. However, as the need for frequency multiplication of the sinusoidal voltage is increased to optimize the wheel skid control, the cost of the electronic equipment required to correct the similarly increased frequency of the signal distortions becomes excessive.